Preheat the oven to 180c/350f.
Add all of the ingredients to a bowl and stir until smooth and well combined.
Spread the batter into a lined baking dish (we used a 17x22cm size, so use similar or a 20x20, if you use larger reduce the cooking time, if smaller then cook longer as it will be thicker).
If using, stir through the choc chips (reserve a few to sprinkle on top).
Bake for 25-30 minutes until a skewer comes out clean, the top is starting to firm and edges are pulling away slightly.
Allow to cool fully in the dish before slicing (they will firm up as they cool).
Store in an airtight container in the fridge for up to 5 days, at room temperature for 2 days, or freeze for up to 2 months.